A. Field of the Invention
This invention relates to an air puff type tonometer for detecting an applanation (flat) state of the cornea of an eye by utilizing a detection light beam that is reflected from the cornea, and measuring the eye pressure by detecting the pressure of the air puff when the applanation state of the cornea is detected.
B. Description of the Prior Art
In a conventional air puff type tonometer, an air puff is directed at the cornea of an eye to be tested and at the same time a detection light beam is projected toward the cornea from a light source. When the detection light beam is reflected from the cornea, the reflected light beam is received by a light receiving device. The applanation state of the cornea is detected based on the quantity of the light received by the light receiving device. In the conventional tonometer, the quantity of light provided from the light source and the detection sensitivity of the applanation detecting system are set constant, because the reflectance of the cornea of the eye to be tested, when the applanation is obtained, is presumed constant.
However, in practice, the reflectance of the cornea may be different from one individual to another. Particularly, when the outer surface of the cornea is not smooth, the reflectance of such cornea can be as small as 1/4 to 1/5 times that of a smooth cornea outer surface.
Therefore, if the detecting sensitivity of the applanation detecting system is adjusted for an eye with a maximum reflectance, the detecting system would not work well for an eye with a minimum reflectance because the detection signal provided from the applanation detecting system becomes too small for an accurate detection of the applanation. On the other hand, if the detecting sensitivity is adjusted for an eye with the minimum reflectance, the applanation detecting system would be saturated if the detection were made for an eye with the maximum reflectance, thus making it impossible to detect the applanation state of the cornea.